Ceramics have excellent heat resistance, corrosion resistance and heat insulating properties in comparison to metal materials, and thus they have received attention as structural materials in place of metals in harsh environments of high temperature, corrosion, etc. However, ceramics cannot be deformed in the same way as metal materials, and stress becomes concentrated on defects in the material and flaws on the surface to lead to easy breakage, giving them the disadvantage of poor fracture toughness. In order to improve this fracture toughness of ceramics, particles or fibers are dispersed in the matrix to absorb the rupture energy.
Methods for the production of fiber reinforced ceramics are largely classified into three types: powder sintering methods, CVD (CVI) methods and impregnation methods.
(1) In powder sintering methods, a reinforcing material such as whiskers or the like is mixed with powder which forms a matrix, and this mixture is sintered at a high temperature of 1600.degree. C. or above. Since powder sintering requires high-temperatures to obtain a consolidated sintered body, its disadvantages include that it can only be used for a limited reinforcing materials.
(2) CVD (CVI) methods involve deposition of a ceramic matrix from a gaseous precursor into spaces in a preform, which is made of inorganic fibers, whiskers or the like. In the CVD (CVI) method, the substance which is to form the ceramic matrix may also be filled into fine pores, but a large number of pores are left as closed pores, and it is generally difficult to fill more than 80% of the pores. Other problems encountered include long production time, high production costs and difficulty of application to large-sized forms due to limits of the apparatuses.
(3) Impregnation methods involve impregnating a polymer which is converted into ceramic after heat treatment, into a preform made of inorganic fibers, whiskers or the like, and then firing it to form a matrix. By repeating the steps of impregnation and heating it is possible to obtain a more consolidated fired body, and this method may be applied to structures with complex shapes and to large-sized forms. In addition, because the firing is performed at a lower temperature than in powder sintering, there is no limit to the type of reinforcing material which may be used.
Ceramic composite materials prepared by thermal decomposition of a ceramic precursor polymer to obtain a matrix are found described in European Patent No. 0,125,722, U.S. Pat. No. 4,117,057 and U.S. Pat. No. 4,460,638. The drawback of these methods is that the polymer must be diluted to about 50% with an organic solvent for its impregnation into the preform. Since the polymer used is a solid or highly viscous liquid, it must be diluted with a solvent to adjust it to a viscosity suitable for impregnation. This solvent must be removed after the impregnation however, and therefore a drying process is necessary, and since pores are left after drying the resulting material has a low density and low strength.
A method designed to improve this is described in Japanese Unexamined Patent Publication (Kokai) No. 2-188471, in which method polysilazane is impregnated without dilution with a solvent. When liquid polysilazane is used, curing and infusibility treatment are necessary after impregnation in order to keep the shape of the product. In this method, the impregnated body is treated for infusibility in an atmosphere of ammonia, urotropin, amine, chlorosilane, or the like. However, when a gas is used in this manner for infusibility, a difference results in the composition of the outer and inner layers of the sample, with the outer layer being more consolidated and densified. Consequently, even if reimpregnation is attempted to improve the density, inner pores are brocked by a outer densified layer, thus lowering the efficiency of densification, and the strength of the final sample is lowered.
Furthermore, in Japanese Unexamined Patent Publication (Kokai) No. 1-305869 (French Patent No. 88 04546) there is described an alternative method of improving the impregnation efficiency, in which a polymerization catalyst is adsorbed onto a preform and then a matrix precursor is impregnated therein and polymerized. However, this method has two drawbacks, one being that the process is complicated and the other that it is required to add, in the form of the polymerization catalyst, an element which is not necessary in the final product and, depending on the case, also harmful.
In addition, there are examples of composite materials prepared using ceramic precursor polymers in K. Ueno, E. Fitzer, et al., Proceedings of the 1988 Annual Meeting of the Nihon Ceramic Society, p.158-159, (1988) and B. Walker, R. Rice, Ceramic Bulletin, p.916-923, 62, (1983), but neither of these provide products with sufficient strength.
As a method of producing fiber reinforced ceramics, the impregnation method has the advantages mentioned above, but also has the following disadvantages in regard to the polymers and steps employed, for which reasons fired bodies with high strength near the theoretical density cannot be obtained.
That is, the polymer preferably possesses the following, but there are no polymers which possess all of these requirement.
(i) The ceramic phase which is converted after heat treatment is heat-resistant, oxidation resistant and corrosion-resistant. PA1 (ii) The ceramic yield is high in order to minimize repetition of the process. PA1 (iii) It has a low viscosity for satisfactory impregnability, and a satisfactory wettability. PA1 (iv) It is preferably a thermosetting polymer. This is for dimensional stability of the product, prevention of bubbling due to the generation of cracked gas, and defects resulting therefrom, and elimination of the time-consuming infusibility treatment. PA1 1 Carbides: Silicon carbide, titanium carbide, zirconium carbide, vanadium carbide, chromium carbide, tungsten carbide, beryllium carbide, boron carbide, and other carbides. PA1 2 Nitrides: Silicon nitride, titanium nitride, zirconium nitride, vanadium nitride, beryllium nitride, boron nitride, aluminum nitride, and other nitrides. PA1 3 Oxides: Alumina, silica, magnesia, zirconia, titania, mullite, cordierite, yttria, borate glass, high-silica-containing glass, silicon oxynitride, sialon, and other oxides. PA1 4 Silicides: Iron monosilicide, triboro monosilicide, hexaboro monosilicide, dimagnesium monosilicide, manganese monosilicide, cobalt silicide, divanadium monosilicide, and other silicides. PA1 5 Borides: Chromium boride, tungsten boride, titanium boride, molybdenum boride, nickel boride, dimolybdenum boride, ditungsten boride, tetraborocarbide, diborotrioxide, and other borides. PA1 P(R.sup.3).sub.3 PA1 [(R.sup.5).sub.2 PN].sub.x PA1 P(R.sup.5).sub.5 PA1 P.sub.2 O.sub.5 PA1 OP(R.sup.5).sub.3 PA1 Viscosity: 100 Pa.s or lower, and preferably 1 Pa.s or lower. PA1 Number average molecular weight: 3000-200, and preferably 1500-400. PA1 Viscosity: 100 Pa.s or lower, and preferably 1Pa.s or lower. PA1 Number average molecular weight: 3000-200, and preferably 1500-400. PA1 Viscosity: 1Pa.s or higher, or solid. PA1 Number average molecular weight: 100,000-200, and preferably 10,000-1000. PA1 (i) Since during vacuum impregnation the pressure cannot be made to lower than the vapor pressure of xylene (about 10 mmHg), the impregnation is incomplete. PA1 (ii) The organic solvent must be removed prior to the firing, and much time is required for this drying step. PA1 (iii) Since the impregnation solution contains a large amount of the organic solvent, the result is a poor impregnation efficiency. PA1 (1) Carbides: Silicon carbide, titanium carbide, zirconium carbide, vanadium carbide, chromium carbide, molybdenum carbide, tungsten carbide, beryllium carbide, boron carbide, and other carbides. PA1 (2) Nitrides: Silicon nitride, titanium nitride, zirconium nitride, vanadium nitride, beryllium nitride, boron nitride, aluminum nitride, and other nitrides. PA1 (3) Oxides: Alumina, silica, magnesia, zirconia, titania, mullite, cordierite, yttria, borate glass, high-silica-containing glass, aluminosilicate glass, silicon oxynitride, sialon, and other oxides. PA1 (4) Silicides: Iron monosilicide, triboro monosilicide, hexaboro monosilicide, dimagnesium monosilicide, manganese monosilicide, cobalt silicide, vanadium monosilicide, and other silicides. PA1 (5) Borides: Chromium boride, tungsten boride, titanium boride, molybdenum boride, nickel boride, dimolybdenum boride, ditungsten boride, tetraborocarbide, diborotrioxide, and other borides. PA1 (6) Other: Carbon
Many gaps may remain after repeated impregnation and firing steps and these lower the strength of the product.
Here, it is the first object of the present invention to improve a polymer suited for the production of fiber reinforced ceramics, and to provide a method for producing high-strength, fiber reinforced ceramics by adding improvements to each of the steps in the impregnation method.
In addition, since ceramic materials are superior for their mechanical properties such as high strength, oxidation resistance, etc., their development as high-temperature materials has been increasingly promoted. However, in the conventional methods the dispersing agents and bonding agents used for molding are scattered during sintering and therefore many gaps appear.
(1) A method has been proposed for achieving greater consolidation by coating an inorganic polysilazane onto the surface of a porous ceramic obtained by a conventional method. The coating of the polysilazane is fired under inert or oxygen-containing atmosphere. This coating prevents the penetration of high-pressure gas into the interior of the porous body under HIP treatment. In this method, the degree of polymerization of the polysilazane is specified to 6-25, in order to form a uniform film on the surface of the porous body.
(2) A method has been proposed for achieving greater consolidation by using a liquid high molecular ceramic precursor as a dispersing agent or bonding agent and converting the ceramic precursor into a ceramic. In order to disperse the ceramic precursor uniformly and minimize the loss of evaporation and decomposition, the viscosity of the precursor is specified at 10,000-1 poise. To obtain further consolidation, the repetition of the impregnation of the precursor is performed.
Nevertheless, the method (1) involves a complicated process. Because of HIP treatment, it cannot be applied to objects with complex shapes.
The method (2) uses an organic-type precursor, which has much weight loss and many free carbon after decomposition, which lower he mechanical strength and properties.
Generally speaking, a more consolidated ceramic material may be obtained by filling the gaps by repeating impregnation and firing with a preceramic polymer. And this may be applied to structures with complex shapes and large-sized forms. Although the impregnation method has such advantages, for obtaining high-density and high-strength, it is necessary to select the polymer to be used and strictly control the steps sintered body.
Here, it is the second object of the present invention to improve a polymer suited for the production of high-density ceramics by the impregnation method, and to provide a method for producing high-strength, fiber reinforced ceramics by adding improvements to each of the steps in the impregnation method.